1. Field of the Invention
The present invention relates to a charge pump step-down circuit which reduces and provides an applied potential, and a power supply circuit which produces a plurality of power supply potentials using the charge pump step-down circuit. The invention also relates to a semiconductor integrated circuit (a liquid crystal display driver) including the step-down circuit or the power supply circuit and a method of driving a liquid crystal display.
2. Related Art
FIG. 4 is a schematic diagram showing a conventional charge pump ½ step-down circuit. This circuit halves a first potential V1 and provides a second potential V2. As shown in FIG. 4, the circuit includes P-channel MOS transistors Q1 and Q2 and N-channel MOS transistors Q3 and Q4 connected in series between the first potential V1 and a power supply potential VSS (referred to here as a ground potential) on the low potential side. The circuit also includes a capacitor C1 connected to a point between a connection point of the transistors Q1 with Q2 and a connection point of the transistors Q3 with Q4, a capacitor C2 connected to a point between the first potential V1 and the second potential V2, and a capacitor C3 connected to a point between the second potential V2 and the ground potential VSS. Here, the capacity of the capacitor C2 is equivalent to that of the capacitor C3.
To gates of the transistors Q1 to Q4, clock signals CKA to CKD, respectively, with wave patterns shown in FIG. 5 are applied. FIG. 6(a) shows an equivalent circuit of the step-down circuit during a period of T1 shown in FIG. 5, while FIG. 6(b) shows an equivalent circuit of the step-down circuit during a period of T2 in FIG. 5.
During the period T1, as shown in FIG. 6(a), the capacitor C2 and the capacitors (C1+C3) are connected in series between the first potential V1 and the ground potential VSS. With both capacitors being charged, the first potential V1 is divided into the ratio of the capacitor (C1+C3) with the capacitor C2. Then during the period T2, as shown in FIG. 6(b) the capacitors (C1+C2) and the capacitor C3 are connected in series between the first potential V1 and the ground potential VSS. With both capacitors being charged, the first potential V1 is divided into the ratio of capacitor C3 with the capacitors (C1+C2). As a result of switching the periods T1 and T2 repeatedly, a voltage that is about half as large as the first potential V1 is maintained at both ends of the capacitor C2. Consequently, the charge pump step-down circuit does not need to pass a direct current loss, and thereby it provides extremely high efficiency.
The charge pump step-down circuit, however, merely forwards electrical charges stored in capacitors. Therefore, it has a problem in that an output voltage is considerably lowered when trying to pull out as large a current as possible.
Meanwhile, an LCD driver includes a power supply circuit that produces some kinds of power supply potentials based on a stabilized power supply potential so as to produce potentials in accordance with input image data and to output them to LCD electrodes. FIG. 7 shows such a conventional power supply circuit. Since this power supply circuit employs a voltage divider including resistors and a voltage buffer including operational amplifiers, it has a problem in that the operational amplifiers, in particular, deliver a direct current loss, which results in high power consumption.
In consideration of the above-mentioned problems, the invention aims to (1) provide a stable potential regardless of variations in a load with a charge pump step-down circuit, (2) reduce a direct current loss of a power supply circuit that produces some kinds of power supply potentials based on an input potential, and thereby lower power consumption, and (3) provide a semiconductor integrated circuit that puts such a step-down circuit or a power supply circuit into practice.